1. Field of the Invention
The present invention relates to a semiconductor device comprising a metallized aluminum alloy layer, which eliminates disadvantages inherent to conventional metallized aluminum layers formed on a silicon substrate.
2. Description of the Related Art
In the process of producing an aluminum layer metallized on a silicon substrate, first, windows are opened through an insulating layer formed on P-N junction areas of the silicon substrate, then, aluminum is metallized thereon by a sputtering process and is patterned by dry etching. In addition, a passivation layer is formed on the patterned aluminum layer by chemical vapor deposition of phosphosilicate glass (PSG).
When the silicon substrate is subjected to a thermal treatment of 400.degree. C. to 500.degree. C. in the processing, such as the formation of a passivation layer, a certain amount of silicon and aluminum dissolves into each other.
The bulky insulating layer forms an edge at the periphery of the window. Consequently, structural strain occurs in the portion of silicon underlying the edge of the insulating layer. The aluminum invades the diffused areas of silicon, particularly at the portion having structural strain, to form so called spikes, which lead to breakage of the p-N junction in the diffused areas. In order to prevent such defects, it has been known to add 1.0% to 2.0% by weight of silicon in the aluminum of the metallized layer.
If an amount of more than 2.0% by weight of silicon is added to the aluminum, the oversaturated silicon precipitates, after cooling. These precipitates form a layer of silicon containing aluminum atoms on the N-type contact area of the silicon substrate. The precipitates act as a P-type conductor, and thus, a parasitic P-N junction is formed at the boundary of the metallized layer of aluminum and the diffused areas of silicon to increase the contact resistance, and relatively large silicon particles are formed within the aluminum layer, remaining even after the dry etching when patterning the aluminum.
The formation of "hillocks" will be explained in detail. When a highly densified electric current is conducted through a metallized aluminum alloy layer at a high temperature hillocks are apt to grow and cause a deterioration in the conductivity of the metallized layer.
The term "electromigration" describes the diffusion of metallic atoms caused by a mutual reaction between the metallic atoms and the densified electron flux which flow through the lattice of the metallic atoms. When an electric current flows, for example, with a current density of more than 5 amperes/cm.sup.2 at a temperature in the range of 150.degree. to 200.degree. C. through a metallized aluminum layer 1.0 .mu.m in depth and 1.2 to 2.0 .mu.m in width, the aluminum atoms gather kinetic energy from the flowing electrons, and dislocate along the direction of the flow of the electrons. Thus, voids are formed in the location lacking the aluminum atoms, and the voids reduce the effective sectional area of the metallized layer and further increase the current density. The area around the voids is locally heated by the joule effect, and the formation of voids is accelerated, which leads to breakages in the metallized layer.
On the other hand, the area into which the aluminum atoms moved becomes rich in aluminum atoms, which leads to a growth of hillocks. The hillocks extrude through the interlayer insulating layer formed on the metallized layer, and lead to breakage of the insulation between the metallized aluminum layer and the upper aluminum layer formed on the interlayer insulating layer.
In addition, hillocks of aluminum extrude into and through a passivation layer formed on the metallized layer, during repeated thermal processing, reducing the moisture resistance of the passivation layer and shortening the service life of the aluminum layer.
Thus, the reliability of a bipolar device, which operates by means of the electric signals, depends on the behaviour of the electromigration. The increasing integration of the semiconductor device means that the size of the metallized layers must be decreased. Therefore, there is a demand for a metallized layer having an excellent resistance against electromigration, which ensures the service life of a metallized layer referred to in this specification.
It has been also known to use metals other than aluminum, in the form of silicide, such as tungsten silicide, as a conductor on a silicon substrate. However, these silicides have the disadvantages of a high electric resistance and difficulties in aluminum wire bonding.